Selection and narrowsense heritability

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From last time... Quantitative traits, concept of
heritability (broad-sense), and interpretation of
heritabilities
Today Evolution of quantitative traits

• Selection and narrow-sense heritability
• Predicting response to selection
• Quantitative traits in natural populations

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Broad-sense heritability (HB) is the proportion
of the phenotypic variability that results from
genetic differences between individuals
Narrow-sense heritability (HN or h2) is the
proportion of total phenotypic variation due to
additive genetic differences between individuals
VA/(VA VD VI VE)
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• Evolution by Natural Selection
• Traits vary.
• Some of that variation is due to variation in
  genetic factors.
• An organisms probability of surviving
  or reproducing depends on the traits it has.
• OUTCOME Change in allele frequencies.

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• Evolution of quantitative traits
• Selection on a trait causes evolution only if
  variation in that trait is at least partly
  genetic.
• Narrow-sense heritability (HN or h2) is the
  keymeasure of genetic variation that determines
  how much evolution occurs during selection.
• If HN 0 for a particular trait, then selection
  has no effect.

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R response to selection S strength of
selection HN heritability (narrow sense)
R HN S
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Can estimate narrow-sense heritabilityby
observing how similar relatives are. ? parent
offspring regression
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Expected regression line if HN 1.0
Estimated HN slope of regression line
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Steeper slopes higher heritabilities
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Parent-offspring regression Beak size in song
sparrows HN 0.98
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Beak size in song sparrows gt no effect of foster
parent phenotype
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R response to selection S strength of
selection HN heritability (narrow sense)
R HN S
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Two different measures of the strength of
selection
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change in trait mean within a single generation
before selection
after selection
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Putting it all together
R HN S
No response (R 0)
Intermediate R
Large R
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Another way to estimate narrow-sense
heritability infer it from how large the
response is to a given bout of selection.
R HN S
HN R/S
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Evolution of a quantitative trait putting HN,
S,and R together into a real example.
Why do tundra populations have larger flowers?
Alpine skypilot (Polemonium)
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Observations --tundra flowers are about 12
larger than timberline flowers --larger flowers
attract more visits from bumblebees --flowers
visited by more bumblebees produce more
seeds Key questions Does selection on flower
size by bumblebees explain whytundra skypilot
flowers are larger? If a new population of
tundra flowers were established fromtimberline
ancestors, how long would it take for flower size
toincrease by 12?
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Heritability of flower size in alpine skypilots
Because Galen knewonly the value for oneparent,
the slope of line(in this case 0.5)estimates
0.5h2.Therefore, HN 1.0
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Selection on flower size in alpine skypilots
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Effects of selection in wild populations
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Nature 426655-658. 2003.
HN 0.69
HN 0.41
Trophy-harvested Unharvested
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Breeding value is the phenotypic deviation of
an individual from the population mean
corrected for age.
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4-year old rams